Method for heat treating in a semimuffle type furnace



IWM W5 J. H. www E72 METHOD FOR HEAT TREATING IN A SEMIMUFFLE TYPE FURNACE Filed Feb. 27, 1959 @TMB/WSW Patented Apr. 8, 1941 METHODFOR HEAT 'FR-EATING IN A SEMI- MUFFLE TYPE FURNACE .James H. Knapp, Beverly Hills, Calif., assigner to James H. Knapp Cb., Los Angeles, Calif., a cor- '.poration of California ApplicationFebruary 27,-1939, Serial No. 258,639

` `1f Claim.

" My invention relates tothe art of `heat treating `metal'and relates more particularly to a method Cwhereby the metal being` heat treated is sur- `rounded with a suitable gaseous atmosphere 'which will not oxidize or decarburize the metal.

' `.It was heretofore common practicein the. art to `heat treat by use of furnaces heated by solid fuel, such as coal and coke, and later by furnaces .using fuel gas or oil, the furnaces being so ,constructed .that the work was enveloped. by the productsof ycombustion of the burning fuel. yThesemgases. of combustionr had a harmful effect l onthe `work andcaused the surfaces to oxidize,

carburZe, or decarburize.

Numerousinventors sought to .eliminate the The/disadvantages VAwere also overcome ing nc products ofcombustion, and by` providing a gaseous enveloping atmosphere in the treating chamber by introducing an especially. propor- .tioned .atmosphere of `products of combustion which would prevent the formation of scale on the work.

It is `an object` of my invention to provide a -direct red or semi-mufliefurnace in which the products of `combustion of thefuel gas` envelop the work being-treated, `and in which there are means for introducinginto the combustion cham- 4.ber substanceswhichwill be chemically converted into suitable gases which will mix with the gases "of combustion and form a substantially neutral gaseous atmospherewhich. will not cause the work to scale.

`It is another object` of: `my invention to provide a, method of heat treatingy in which the products .i (1f-combustion oftbefuel burned to-heat the work l. are caused to envelop `theworkandwhich contain vthe necessary constituents in proper amounts to `prevent scalefrom forming` on the work.

It is another object of my inventionto provide afmethod of heat treating in which the fuel gas and cXygenbearing--gas-arecombusted to produce heat for heating the work to-be heat treated,

ff and in which the combustionr takesvplacein the `presence of a hydrocarbongas andi/or ammonia or other hydrogen `producing gas to produce a resultant mixture of gases which forms a gaseous envelope which is caused to envelop the work for heating the same and for preventing scale.

Other objects of my invention will be brought out during the course of the followingdescription of apreferred embodiment of my method and apparatus. It should be understood that various changes in substances and proportions andpreoise steps maybe made within the principle of my invention and without departing from the spirit and scope thereof `as defined in the appended claims.

In the drawing:

Fig, 1 is a vertical sectional view of a` furnace incorporating the featuresk of my invention, this View being taken as indicated by the arrows l-I of Fig. 2.

- Fig. 2 is a section taken as indicated by the arrows 2--2 of Fig 1 Referring to the drawing` in detail, there is a suitable frame structure Il which supportsthe furnace. The furnace has a floor I2, sidewalls I4., a rear wall l5, a front wall I6, and a roof Il. The front wall I6 is provided with a doorway or entrance I9 lwhich vmay be closed by a doorA 20.

Supported within therfurnace is a hearth 2l which is supported on suitable supporting `blocks 22. This provides theinterior ofthefurnace with a treating chamber 24, a combustion chamber 25, and passages 26 which connectthe heating and combustion chamberslll and 25.

lExtended inwardly into openings 2l formedin the side walls I4 are burners` 28 which` areconnected by pipes29 to a manifold 30, in theV form of the invention illustrated there being two. burners 28 Von each .side of the furnace. ,Connected to the manifold 30, as illustratedin Fig.` 2, is a supply line 3l which is connected to av blower 32 which supplies air or other oxygen bearing..gas in amounts determined byv the ,positionjofthe air valve 33. For the purposeV of supplying fuel gas to the line 3l there is provided in` this line 3| a Venturi meansd to which a ,fuel gas pipe 35 having a fuel gasfvalve 36 is connected. `For the purpose of supplying a hydrocarbon gas. or other carbon monoxide. forming gasand for. supplying .ammonia or other hydrogen `forming gas, I provide a Venturi means 38 to which a pipe` 39 is connected, this pipe 39v having ahydrocarbon gas branch 4U and an ammonia `branch 4l, these branches havingvalves 42 .and 43,- respectively.

`The method of myinvention. is performed as follows.: The. blower 22 is placed in operation and the valves 33 and 36 are openedin amounts to supply a predetermined mixture of combustible and oxygen bearing gases in amounts whichwill produce the desired temperature in the heat treating chamber 24. This combustible mixture is conveyed through the line 3l to the manifold 30 and thereafter passes through the burners 28 intothe chamber 25 where it is ignited and combustion takes .place so as to produce heat and gases or products of combustion. These products of combustion pass upwardly through the passages 2B into the heat treating chamber 24 and thus envelop the work being heat treated.

In the combusting of ordinary fuel gases the productstof combustion may contain CO2, CO, H2O, O2, CH4, Nz, and Hz, depending upon the nature of the fuel and the fuel air ratio employed. Under certain conditions the Oz, CO2, and H2O would produce scaling or oxidization of the work being heat treated, CO and CH4 would produce carburization, and O2, CO2, and H2 would produce decarburization, while CO and H2 would produce a reduction of oxides. It will be understood that the gases present and the proportions of gases present in the products of combustion depend upon the fuel gas which is used and upon the fuel gas-air ratio. Where there is a high fuel gas to air ratio, products of combustion ordinarily contain CO2, CO, H2O, CH4, N2', and only a slight trace of O2, whereas with a lower fuel gas to air ratio there is ordinarily present greater proportions of CO2 and O2 with H2O and N2 and possibly no CO or CH4. It is generally considered preferable toI set the fuel gas to air ratio at a proportion to obtain the highest possible CO2 and to have present a small percentage of CO but with the lowest practical percentage of O2. v

In the practice of my process I add the necessary constituents to the gases of combustion to gas, such as propane, and the valve 43 is opened to supply a hydrogen forming gas, such as ammonia. 'Ihe hydrocarbon gas under the action of heat in the combustion chamber breaks down and supplies the necessary carbon to form carbon monoxide and some hydrogen, and the ammonia breaks down and supplies hydrogen and nitrogen. To increase the carbon monoxide content of the enveloping gaseous atmosphere which is intro-l duced into the treating chamber, more hydrocarbon gas is used, and to increase the hydrogen content, more ammonia is used. It will be understood that the main function of the propane In the process or hydrocarbon gas is to furnish carbon for the production of carbon monoxide and that other hydrocarbon gases, such as butane, methane, ethane, etc., may be employed or, in lieu thereof, carbon may be furnished by a partial burning of solid carbon, such as charcoal.

may be employed or, if desired, hydrogen gas may be introduced.

I prefer to introduce these gases to the combustible mixture so that a thorough mixture will be had when the gases pass through the manifold 30 and burners 28. This may be conveniently done, since the volume of the gases which are added are very slight in comparison with the volume of fuel gas and air` these added gases may be introduced directly into the combustion chamber where they will be subjected to the action of the heat without premixing with the combustible gases and will be converted into carbon monoxide and hydrogen,

depending upon the amounts of gases so intro-v|`4 duced.

It has been found that the percentages of CO and H2 furnish a satisfactory index for the In place of am-v monia other gases which will produce hydrogen f:

If desired, however, y

nature of the gaseous atmosphere in the heat treating chamber and satisfactory results may be obtained at various temperatures with percentages of these gases ranging from 4% to 30% by volume. In actual practice in a small semimufle tool furnace of the type shown in the drawing best results have been obtained over a temperature range of from 1400" F. to 2300 F. with approximately 5% by volume of CO and 25% by volume of H2, while with percentages of approximately 10% CO and 10% H2 or other variations of the above percentages good results have been obtained, but in the experiments conducted the results with these latter mentioned percentages were found to be limited to lower temperature ranges. The amounts of hydrocarbon gas and ammonia or its equivalent introduced may be readily varied by adjusting the valves 42 and 43, and these proportions may be varied in order to obtain the desired percentages of carbon monoxide and hydrogen so as to obtain a balanced or neutral gaseous atmosphere in the heat treating i chamber.

I have found that for the lower temperature ranges in the heat treating chamber 24 of between 1400 F. and 1600 F. with certain fuel gases the fuel gas to air ratio can be set high .enough in fuel gas so as to produce suflicient CO in the gaseous atmosphere, and that it is only necessary to add ammonia or other hydrogen producing gas to obtain the necessary hydrogen percentage. However, in higher temperatures of from 1600 F. to 2300 F., such a fuel gas-air ratio can not produce the necessary combustion temperature and therefore can not be economically used for these higher temperature ranges, and that, therefore, for these higher temperature ranges it is necessary to add the hydrocarbon gas. It should be understood, however, that my invention is applicable for use where any of the heat treating temperatures are employed and more satisfactory results, both from the type of gaseous atmosphere which is produced and from the standpoint of economy, can be obtained where the hydrogen gas and/or ammonia are added in accordance with the teachings of my process.

In the foregoing description I have disclosed the apparatus of my invention and the preferred process of my invention and have indicated the various elements and percentages which have been found satisfactory. It should be understood, however, that my invention is not limited to the details disclosed herein but extends to all apparatus and all processes incorporating the principle thereof and which fall within the spirit and scope of the appended claim.

I claim as my invention:

In a method of heat treating which includes the steps of: mixing with predetermined proportions of a combustible mixture of combustible and oxygen bearing gases hydrocarbon gas and hydrogen producing gas; combusting the combustible mixture to producevheat and products of combustion and to convert by the action of heat the hydrocarbon gas and hydrogen producing gas into carbon monoxide and hydrogen and thereby form a substantially non-scaling atmosphere; and enveloping the work to be heeft treated with said gaseous atmosphere to heat and prevent scaling of the work.

JAMES H. KNAPP. 

